Ultraviolet photoconductivity of amorphous ZnAlSnO thin-film transistors

Abstract

Ultraviolet (UV) photoconductivity of solution-processed amorphous zinc-aluminum-tin oxide (a-ZATO) TFTs has been investigated in detail to clarify the device responses. The sensitivities of a-ZATO TFTs towards UV light are strongly dependent on the Al content. The UV exposure and recovery process are decided by the state changes of oxygen defects (V⁰_O and V_O²⁺). An appropriate Al addition can not only reduce the amounts of photogenerated carriers by increasing the band gap of ZATO films, but also markedly decrease the UV photoconductivity and recovery time, which benefits the TFT applications in the display fields. Based on UV behaviors of ZATO TFTs, a feasible and reasonable mechanism model has been proposed, which provides a fundamental insight to UV photoconductivity of ZATO TFTs and may also be applied to other AOS TFTs.

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Recently, wide-band gap metal oxide semiconductors, especially amorphous ones, have attracted great attention in the fields of transparent displays, electronic and electro-optic components.^1–5 With high mobility, excellent optical transparency and long-term stability, amorphous oxide semiconductors (AOSs) are commonly used as thin-film transistor (TFT) channels to realize high pixels, high frame rate and high resolution displays.^6,7 Up to now, amorphous indium-gallium-zinc oxide (a-IGZO) and amorphous zinc-indium-tin oxide (a-ZITO) have been the most attractive materials for practical applications.^8–10 The above attentions on AOSs are almost all on electro-optic and display devices. However, AOSs can also be used as effective phototransistors and photosensors, which is a very potential and promising research area.¹⁰ Compared to the conventional photodiodes, AOS phototransistors exhibit much larger sensitivity, lower noise and better characters for integrated circuit, and these advantages deserve further investigation.^11,12

In our previous reports, we have demonstrated indium-free amorphous zinc-aluminum-tin oxide (a-ZATO) TFTs with excellent electrical performances and long-term stability,¹³ which are very promising for commercial applications in next-generation displays. In practical applications, most of their uses will be exposed to a backlight or ambient light during operation, so the light sensitivity of ZATO TFTs is critically important. In addition, for a wide range of functional applications such as future phototransistors, it is also very necessary to have a precise knowledge of the effect of light on device behaviors. In this letter, we conduct a study of ultraviolet (UV) photoconductivity in a-ZATO TFTs. The UV effects on performances of a-ZATO TFTs are investigated in detail. A mechanism model is proposed to reveal the photoconductivity and device stability during operation. This work provides a feasible model to explain UV behaviors of a-ZATO TFTs. More importantly, this model can also be used to other AOS TFTs for understanding the UV responses.

The ZATO precursors were prepared by a combustion solution process with mixing the single precursor.¹³ The channel films were made on n⁺⁺ Si/150 nm SiO₂ wafers (University Wafer) by spinning coating using a combustion solution process. 100 nm Al films prepared by electron beam evaporation were used as the source and drain electrodes. The TFT channel width and length were 1000 and 200 μm, respectively. Details could be found in our previous report.¹³ Here, the Al content (in molar ratio) is 0, 0.5, 1.0 and 2.0, thus the samples are correspondingly designed as ZATO (0), ZATO (0.5), ZATO (1.0) and ZATO (2.0), respectively.

The electrical properties of the a-ZATO films were carried out on an HL5500 system using a four-point probe van der Pauw configuration. The optical transmission through film was performed with a UV 3600 ultraviolet-visible spectrophotometer from 225 to 800 nm. Device electrical performances were measured by a source-meter system (Agilent Technologies E5270B) under dark and UV (365 nm, 60 mW cm⁻²) conditions. During the measurement, ZATO TFT devices were exposed to UV illumination and the electrical performances were carried out under UV (30 s, 2 min, 5 min, 10 min and 20 min) and after removing UV (10 s, 100 s, 500 s, 1000 s, 5000 s and 10 000 s).

Fig. 1 depicts the I_DS–V_DS (drain current–drain bias) characteristics curves of a-ZATO (1.0) TFT under dark and 365 nm UV illumination (20 min) at V_GS = 0 V (V_GS is the gate bias). In both cases, I_DS increases as V_DS is elevated. At V_DS = 20 V, I_DS = 3.68 × 10⁻¹⁰ A under dark, which increases to 5.39 × 10⁻⁷ A under UV illumination. Similar data are observed at V_DS = −20 V. The resistance of the ZATO film is 1.25 × 10⁹ Ω cm under dark, which decreases to 6.42 × 10⁵ Ω cm after UV illumination. These results confirm that the UV light (365 nm) can effectively promote the photoconductivity behaviors of the a-ZATO channels. The obvious increment of I_DS indicates that a-ZATO films are sensitive to the UV light and can be used as the phototransistors.

Fig. 1 I_DS–V_DS characteristic curves of a-ZATO (1.0) TFT under dark and 365 nm UV illumination at V_GS = 0 V.

Fig. 2 shows the typical output characteristics of the a-ZATO (1.0) TFT under dark, UV illumination (2 min) and recover periods (10 s, 1000 s) after UV illumination. Other related output curves are shown in Fig. S1 (ESI†). For all the TFT devices, good ohmic contacts at the Al/channel interface have been formed due to no current crowding at low V_DS. A clear pinch-off and current saturation are observed. Moreover, the saturation current of I_DS markedly increases as the positive V_GS increases, indicating the devices operate in an enhanced mode with an n-type channel. Under dark condition, the a-ZATO (1.0) TFT exhibits a saturation current of 15.14 μA, while it increases to 47.88 μA after UV illumination for 2 min at V_GS = 40 V. After removing the UV illumination, the saturation current decreases to 32.71 and 18.49 μA for recover durations of 10 and 1000 s, respectively. Therefore, it is believed that the photoelectric effect by UV contributes to the saturation current changes.

Fig. 2 Typical output characteristics of a-ZATO (1.0) TFT: (a) under dark, (b) under UV illumination for 2 min, (c) 10 s recover after UV exposure, and (d) 1000 s recover after UV exposure.

Here, a-ZATO TFTs with different Al contents (0, 0.5, 1.0 and 2.0) are used to investigate the UV photoconductivity.¹⁴ Fig. 3(a), (c), (e) and (g) show the I_DS–V_GS curve changes of a-ZATO (0, 0.5, 1.0 and 2.0) TFTs under dark and UV illuminating (2 min and 20 min) at V_DS = 10 V. At off-state (V_GS < V_ON, V_ON is the open voltage), I_DS markedly increases more than 1–5 orders of magnitude under UV illumination as compared to that measured under dark, which is dependent on the Al content in the ZATO channels. Notably, the increased magnitude of I_DS under UV illumination gets much smaller as the Al content increases. This may be due to the enhanced optical energy band by Al addition, which makes the photogenerated carriers more difficult to produce. However, at on-state (V_GS > V_ON), UV exposure brings small changes of I_DS. In order to further study the recover changes after removing UV, various recover time (10 s, 100 s, 500 s, 1000 s, 5000 s and 10 000 s) are adopted to investigate the transfer characteristic. The corresponding recovering I_DS–V_GS curves are shown in Fig. 3(b), (d), (f) and (h). After removing the UV light (e.g., for 10 s), the off-current decreases quickly and the on-current slowly recovers to the original levels. With prolonging the recovering time, the device properties get slowly to recover back. After 10 000 s, all the a-ZATO TFT devices recover to that of the dark conditions. Moreover, the recover speed gets quickly with Al content increasing. For ZATO (0) sample, the off-current recovers to the dark level of 10⁻¹⁰ A after 1000 s. With increasing Al content to 0.5, 1.0 and 2.0, the recovering time of the off-current to the dark conditions are decreased to 100 s. For these samples, with continuously increasing the recovering time (>1000 s), the off-current and on-current both show little variations, the V_ON shifts to be positive direction. Additionally, increasing Al addition results in the much quicker recovery of the V_ON.

Fig. 3 I_DS–V_DS curves of a-ZATO TFTs at V_DS = 10 V: (a) and (b) for ZATO (0), (c) and (d) for ZATO (0.5), (e) and (f) for ZATO (1.0), and (g) and (h) for ZATO (2.0) at UV illuminating and post recovering for different time (10, 100, 500, 1000, 5000, and 10 000 s), respectively.

Table 1 summarizes the changes of V_th and density of the interfacial trap states (N_t) at the ZATO channel/SiO₂ interface of a-ZATO TFTs under UV illumination and recovering periods. The V_th and N_t can be determined using the following equations:

where C_i, μ_FE, W, L, V_th, SS, e, K, T, and q are the capacitance per unit area of the gate insulator (0.023 μF cm⁻²), field effect mobility, channel width, channel length, threshold voltage, subthreshold swing, Euler's constant, Boltzmann constant, temperature, and quantity of one electron, respectively.

Table 1 V_th and N_t (×10¹² cm⁻²) changes of a-ZATO (0, 0.5, 1.0 and 2.0) TFTs under UV illumination and the recovering period (dark, 10 s, 100 s, 500 s, 1000 s, 5000 s and 10 000 s) after UV illumination

	Samples	Dark	UV 2 min	10 s	100 s	500 s	1000 s	5000 s	10 000 s
Vth	ZATO (0)	−0.13	−31.10	−21.14	−12.32	−3.82	−2.08	−0.56	−0.56
	ZATO (0.5)	5.32	−25.64	−5.90	−1.72	0.49	1.20	3.31	4.28
	ZATO (1.0)	12.19	−10.57	1.34	8.18	10.56	10.67	11.82	12.20
	ZATO (2.0)	6.66	−30.45	−3.75	3.17	5.22	6.17	6.43	6.54
Nt	ZATO (0)	1.48	40.67	28.77	14.69	3.78	2.00	1.09	1.08
	ZATO (0.5)	1.14	23.15	7.28	2.55	2.53	2.42	2.31	1.24
	ZATO (1.0)	0.77	16.13	9.62	2.66	1.88	1.29	1.04	0.97
	ZATO (2.0)	1.59	18.57	9.16	5.42	2.63	2.61	1.41	1.42

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The corresponding μ_FE and device response time (τ = R_ON × C_i) are shown in Tables S1 and S2 (ESI†). Here, we focus on the recover properties of the same TFT. As shown in Table 1, during the UV illumination, the V_th shifts to a very negative value compared with that of the dark sample: from −0.13, 5.32, 12.19, and 6.66 V to −31.10, −25.64, −10.57, and −30.45 V for a-ZATO (0), a-ZATO (0.5), a-ZATO (1.0), and a-ZATO (2.0), respectively. However, after removing the UV sources, all TFTs exhibit a tendency that the V_th recovers to the dark value slowly with prolonging the recovering time. After 5000 s, the V_th values of all the samples recover to the original states except for small deviation. Additionally, the μ_FE and N_t values increase during the UV illumination, and then decrease slowly to the original states after removing the UV sources. While, for τ, the tendency is opposite compared with that of μ_FE changes. Therefore, we can see that an appropriate Al addition can do well for the TFT devices and UV detections.

Fig. S2† shows optical transmittance spectra of a-ZATO films with various Al contents. In general, the amorphous semiconductor is commonly a non-direct semiconductor. The optical band gap energies (E^opt_g) can be calculated by the Tuac relation,¹⁵

αhν = C(hν − E^opt_g)^m (3)

where h is Planck's constant, ν is the photon frequency, α is the absorption coefficient, and C is a constant. The value of m is 1/2 for a direct semiconductor and 2 for a non-direct semiconductor. The amorphous semiconductor is commonly a non-direct semiconductor. However, in order to determine the difference of band gap energies of an amorphous semiconductor, the m value induced by the direct semiconductor model has been usually used. Thus, we apply m = 1/2 for our calculation.¹⁵ The E^opt_g values are displayed in the inset of Fig. S2,† which increase from 3.04 to 3.59 eV as the Al content increases from 0 to 2.0. The larger optical band gaps can not only enhance the barrier of the electron transition, thus decreasing the carrier concentration,¹³ but also affect the UV photoconductivity of a-ZATO TFTs.

Based on the V_th, N_t, μ_FE and τ changes during the UV illumination and UV recover periods, we have proposed a mechanism model to illustrate the UV behaviors of a-ZATO TFTs. As Fig. 3 depicts, I_DS markedly increases upon UV illumination during the off-state. Moreover, with increasing V_GS from −20 V to the V_ON, I_DS continue increasing. As shown in Fig. 4(a), in this process, electrons are mostly emitted into conduction band (E_c) from the valence band (E_v) by absorbing the UV photo energy (3.40 eV), which is larger than the band gap of ZATO films when x ≤ 0.5 (3.04 eV for ZATO (0) and 3.23 eV for ZATO (0.5)). While x ≥ 1.0, this direct electron transition will not occur due to the band gap is larger than UV energy (3.48 eV for ZATO (1.0) and 3.59 eV for ZATO (2.0)). Moreover, previous studies confirm neutral oxygen vacancies (V⁰_O) always exist with a high density at the top of E_v to form deep levels.^16,17 The V⁰_O can be photo-excited to release two electrons to E_c by V⁰_O → V_O⁺ + e and V_O⁺ → V_O²⁺ + e.¹⁶ Therefore, for x ≤ 0.5, UV generated electrons in the band gap and V⁰_O contribute to the large I_DS. However, the electron transfer from E_v to E_c is the dominated reason for promoting photoconductivity in this case. For x ≥ 1.0, the photo-excited V⁰_O electrons dominate the I_DS enhancement, which makes the photoconductivity promoted. Moreover, we deduce these different reasons lead to the divergence of the I_DS–V_GS curves at off-state between the samples of x ≤ 0.5 and x ≥ 1.0. The persistent increment of I_DS with increasing V_GS at off-state is due to the electron accumulation at E_c. At off-state, the electrons tend to escape from the TFT electron transfer channel (close to the channel/SiO₂ interface) to the ZATO matrix, thus increasing the electrical properties of ZATO films and making the TFTs lose the transfer characteristics.¹⁸ However, at on-state (V_GS > V_ON), small changes of I_DS occur due to that the gate-bias-induced charge carriers dominate the photo-induced carriers, as shown in Fig. 4(b). The divergence of I_DS values between the dark and UV conditions at on-state are resulted from the UV generated accumulation. Moreover, the intrinsic shallow centers between E_f and E_c can also act as electron trapping centers at on-state.⁸ At on-state, all the electrons are accumulated close the channel/SiO₂ interface to form the electron transfer channel, which realizes the various electrical responses.¹⁸

Fig. 4 Schematic diagram of mechanism model to illustrate the UV photoconductivity of a-ZATO TFTs.

After removing the UV sources, a persistent photoconductivity is observed, as shown in Fig. 4(c). Previous studies confirm that the excited O vacancies lead to the persistent photoconductivity in direct semiconductor. However, in AOS systems, the carrier recombination is much complex than crystalline semiconductor oxides due to the non-direct characters. For a-ZATO TFTs, we deduce four factors contribute to the carrier recombination:

1. Direct recombination of electrons at E_c and holes at E_v. Due to the non-direct character for a-ZATO AOSs, the direct recombination of electrons and holes needs the participation of lattice vibration. Moreover, Al addition can enlarge the band gap, which further shortens the recombination time. This is the reason for quicker recover of transfer characteristics with Al increasing, as shown in Fig. 3(b), (d), (f) and (h). Additionally, Al addition can decrease the V_O in some extent, which will affect the lattice vibration.

2. V⁰_O can form the ionized V_O²⁺ by capturing the photo-excited holes through V⁰_O + 2h → V_O²⁺, which will reduce the direct recombination. These V_O²⁺ can exist stably in the ZATO films. In the recovery, the photo-excited holes of high-content Al samples (x ≥ 1.0) are less than that of small content Al samples (x ≤ 0.5). Therefore, less time is needed to recombine hole during the recovery for high Al content.

3. Some photo-excited V_O²⁺ during the UV illumination can return to the ground state (V⁰_O) by thermal activation or other energy.^19–22 The existence of V_O²⁺ in the matrix can also combine the photo-excited electrons to form the ground state (V⁰_O). The amounts of V_O²⁺ and photo-excited electrons result in the difference for various a-ZATO TFT recovery.

4. In AOS, there always exist some shallow levels to trap the electrons close to the ZATO/SiO₂ interface.^23,24 During the recovery, the trapped electrons need long time to return to low energy level by relaxation. Thus, some trapped electrons are easily combined with the photo-excited holes to complete the recombination.

In summary, a-ZATO TFTs have been fabricated using a solution process for the UV investigation. The UV photoconductivity of a-ZATO TFTs have been investigated in detail. Results indicate that the sensitivities of a-ZATO TFTs towards to UV light are determined by the Al content. The Al content and O vacancies including V⁰_O and V_O²⁺ play a crucial role in the UV illumination and the recover process. An appropriate Al addition can markedly decrease the UV photoconductivity and recovery time, which is very beneficial for the TFT applications in the display fields. Moreover, the short recovery time is also a positive sign in UV detection with fast response. A feasible and reasonable mechanism model has been proposed to illustrate the TFT UV behaviors. This proposed model can give a fundamental sight of UV photoconductivity of AOS TFTs.

Acknowledgements

This work was supported by National Natural Science Foundation of China under Grant no. 51372002 and 51302021.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra09670g

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